The present invention relates to material handling vehicles and, more particularly, to a technique for slowing and stopping material handling vehicles by using an efficient plugging technique, and one which provides safety by monitoring braking performance.
Mechanical brakes are conventionally used to slow and stop material handling vehicles, such as forklift trucks. The brakes can be used for service, for parking, and/or for emergency braking. Braking is accomplished by means of a mechanical or mechanical/hydraulic linkage coupling between the deadman's pedal and the brake. Braking is generally variable and proportional to the distance that the deadman's pedal is lifted.
When the electric traction motor itself is used to slow the vehicle, however, a technique called "plugging" is most often used. Plugging is accomplished when the operator moves the speed control mechanism such that power is applied to the motor tending to drive the motor in a direction opposite to that in which the vehicle is moving, although the motor still rotates in the forward direction. The motor operates, in both the plugging and driving modes, by the operator actuating the control handle, which generates both positive and negative torques. Until the vehicle slows to zero speed, the motor acts as a dynamic brake. Ultimately, the vehicle slows to a stop and begins to move in the reverse direction, at which time the plugging action ceases. It should be noted that plugging, as herein defined, may utilize the principle of dynamic braking as well as a combination of regenerative and dynamic braking.
The present invention seeks to improve the aforesaid braking effort by achieving consistent and predictable (factory programmable) braking. In this way, the wear of the mechanical brake is reduced, thus extending its operating life, while reducing the need for repeated, frequent servicing. The mechanical brake is used for parking purposes only and as a backup to the motor braking. Also, since non-variable ON-OFF brakes are less expensive than are variable type brakes, overall costs are reduced.
The traction motor is used not only for driving the vehicle, but for plug braking and for service braking. In service braking, the operator initiates negative torque in the motor by releasing the deadman's pedal or switch. An electrical mechanism (specifically, plugging) is provided for coupling between the deadman's pedal and the control system to generate negative torque. The service braking effort cannot be altered by the user in the present invention, since it is intended to bring the vehicle to a full stop within applicable safety standards.
The inventive procedure improves the braking effort by terminating plugging at or near zero velocity, and only then engages the mechanical brake. The system programs the braking action to cut-in the mechanical brake system at the optimum time. Efficiency is enhanced also by slowing the vehicle at a predetermined plug rate.
The invention improves safety by interrupting the controlled rate of plugging in the event that the deadman's switch is thrown. In such a case, the rate of deceleration is increased to provide service braking. Safety is further ensured by detecting if service braking is actually occurring. If it is not, the mechanical parking brake, which is normally used only for parking, could be engaged as an emergency brake to stop the vehicle when the motor fails to brake.
Service braking requires the proper operation of many components such as the deadman's pedal switches, the microprocessor control system, the field drive circuits, the free-wheeling diode circuit, and the motor itself. Each of these components could be tested individually by a properly designed control system. However, a failure situation must be detected as soon as possible. Since all possible failure modes must be tested for this system to be effective, individual testing of all of these components would place an undue burden on the control system and/or limit its ability to react quickly.
Alternatively, it would be advantageous to provide a means to determine whether service braking is actually occurring by observing the end results.
It would also be advantageous to detect service braking by more than one method to provide redundancy. For this reason, two different methods of detecting plugging during service braking are described herein. The first method involves measuring the deceleration of the vehicle; the second involves measuring the electrical current in the drive motor armature during plugging. Testing armature current in order to detect plugging is especially effective because the amount of current is a direct indication of the effort exerted by the motor and its anticipated deceleration. These tests Call be run concurrently to provide a high level of reliability.
It would also be advantageous to use the same sensing elements as the main control system in the tests of plugging in order to reduce costs and to provide a means of cross-checking operation.
It would also be advantageous to provide a plugging detection system capable of reacting as quickly as possible to a failure of service braking.
U.S. Pat. No. 4,518,902 issued to Melocik et al discloses the conventional plugging technique described above. The Melocik reference teaches the use of a circuit for controlling the direction of motor energization, a circuit for generating digital numbers representing motor speeds, a circuit for generating a data signal representing a plugging condition, a programmable processor for generating pulse trains, and a power transistor for coupling power to the motor in response to the pulse trains. This patented system pulses the motor at a preselected plugging rate by means of a feedback control. Plugging continues until the vehicle is stopped.
U.S. Pat. No. 4,265,337 issued to Dammeyer discloses a speed control circuit for a lift truck driven by an electric truck drive motor. A motor controller initiates the plugging technique. The Dammeyer reference requires two switches to actuate motor plugging. Moreover, when the vehicle stops, the motor continues in the reverse direction, which can cause the truck to move in a reverse direction.
It would be advantageous to provide an improved system for slowing and stopping a vehicle by means of its service motor in order to reduce the cost and wear on the mechanical brake.
It would also be advantageous to automatically engage the motor braking technique when the deadman's switch is released.
It would also be advantageous to terminate service braking if the deadman's pedal is engaged.
It would also be advantageous to sense when the vehicle is slowing down to a predetermined rate of speed and then to disengage the motor, so that the mechanical brake can be engaged and, eventually, used as a parking brake.
It would also be advantageous to programmably control deceleration of the vehicle.